This invention relates to a low-viscosity offset printing ink which essentially comprises a binder, additives and 5-40% by weight of pearlescent pigments
Offset printing inks generally comprise binders, pigments or dyes and additives. In the case of print products for packaging prints, labels and high-quality magazines, a frequent requirement is the imparting of high gloss to the printed articles.
All offset printing inks containing pearlescent pigments have a disadvantage in that they have problems with stability during continuous printing. They tend to accumulate rapidly on the inking system, printing plate and blanket, so that problem-free continuous printing of more than 10,000 sheets is generally impossible. Furthermore, the glossing of such prints is generally unsatisfactory, due to the inadequate amount of pigments transferred to the print products. The concentration of pigment in the ink drops over the transport path over inking system, plate and blanket. The pigment accumulates at exposed areas on the plate and blanket and results in piling and caking. In general, only pearlescent pigments having a very small mean particle size are suitable, since the particle size is critical for pigment transfer in offset printing. Such pigments only exist for pearl-white and pastel-shaded inkings, but not for gold, silver, bronze and copper shades, in which there is a great deal of interest. It has hitherto not been possible to produce such shades satisfactorily in the offset method using pearlescent pigments.
The use of bismuth oxychloride pigment platelets in order to achieve a pearlescent effect likewise gives unsatisfactory results, since the pigment particles are very sensitive to mechanical shear forces and are ground down in the ink nip.
The use of pearlescent pigments in printing inks for offset printing is described in DE 29 03 212, which discloses the pigmenting of a commercial oil-based printer's varnish containing a preferably very finely divided pearlescent pigment. The offset printer's varnish preparation described therein is distinguished by the fact that the proportion of pearlescent pigments is very high, the upper limit of the pigment concentration in the suspension essentially being limited only by the mixture flowability required. The proportion of pearlescent pigments is in the range up to 65% by weight. Varnishes having such a high proportion of pigments are very viscous and may have to be made more flowable using a thinner so as to be processable in conventional offset printing machines. Experiments have shown that, in contrast to the teaching of DE 29 03 212, the transfer of pigment from inking system to the substrate is adversely affected at such high pigment levels. Problems such as piling and caking occur during continuous printing, i.e. the achievable pearlescent effect cannot be optimized simply by using a high level of pigments. Furthermore, the ink systems of DE 29 03 212 generally have inadequate dot sharpness, so that the ink systems described therein are essentially restricted to printing on full-color areas.